SQL> create table t1(id int); Table created. SQL> create table t2(id int); Table created. SQL> SQL> insert into t1 values (1); 1 row created. SQL> insert into t2 values (1); 1 row created.In a nutshell, I created two tables that have one row with a numeric 1 value. Let's now run the following query:
SQL> select query_type 2 from (select 'A' query_type 3 from t1 t11 4 where exists ( 5 select null 6 from t2 t21 7 where t21.id = t11.id) 8 union all 9 select 'B' query_type 10 from t1 t12 11 where not exists ( 12 select null 13 from t2 t22 14 where t22.id = t12.id) 15 ); Q - AThen, I am going to put that query into a CURSOR expression:
SQL> select cursor( 2 select query_type 3 from (select 'A' query_type 4 from t1 t11 5 where exists ( 6 select null 7 from t2 t21 8 where t21.id = t11.id) 9 union all 10 select 'B' query_type 11 from t1 t12 12 where not exists ( 13 select null 14 from t2 t22 15 where t22.id = t12.id) 16 ) 17 ) data 18 from dual; DATA -------------------- CURSOR STATEMENT : 1 CURSOR STATEMENT : 1 Q - BThat is when it gets interesting. Somehow the query which returned 'A' being run separately starts returning 'B' after it comes as an input subquery into a CURSOR expression.
I initially started looking at the execution plan in an attempt to figure out what changed in the execution plan between 12.2 and 19c. There are some changes but nothing that drew my eye.
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 2197603499
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 14 (0)| 00:00:01 |
| 1 | VIEW | | 2 | 6 | 12 (0)| 00:00:01 |
| 2 | UNION-ALL | | | | | |
|* 3 | FILTER | | | | | |
| 4 | TABLE ACCESS FULL| T1 | 1 | 13 | 3 (0)| 00:00:01 |
|* 5 | TABLE ACCESS FULL| T2 | 1 | 13 | 3 (0)| 00:00:01 |
|* 6 | FILTER | | | | | |
| 7 | TABLE ACCESS FULL| T1 | 1 | 13 | 3 (0)| 00:00:01 |
|* 8 | TABLE ACCESS FULL| T2 | 1 | 13 | 3 (0)| 00:00:01 |
| 9 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SET$1 / from$_subquery$_001@SEL$2
2 - SET$1
3 - SEL$3
4 - SEL$3 / T11@SEL$3
5 - SEL$4 / T21@SEL$4
6 - SEL$5
7 - SEL$5 / T12@SEL$5
8 - SEL$6 / T22@SEL$6
9 - SEL$1 / DUAL@SEL$1
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
FULL(@"SEL$6" "T22"@"SEL$6")
FULL(@"SEL$4" "T21"@"SEL$4")
PQ_FILTER(@"SEL$3" SERIAL)
FULL(@"SEL$3" "T11"@"SEL$3")
PQ_FILTER(@"SEL$5" SERIAL)
FULL(@"SEL$5" "T12"@"SEL$5")
NO_ACCESS(@"SEL$2" "from$_subquery$_001"@"SEL$2")
OUTLINE_LEAF(@"SEL$1")
OUTLINE_LEAF(@"SEL$2")
OUTLINE_LEAF(@"SET$1")
OUTLINE_LEAF(@"SEL$5")
OUTLINE_LEAF(@"SEL$6")
OUTLINE_LEAF(@"SEL$3")
OUTLINE_LEAF(@"SEL$4")
ALL_ROWS
DB_VERSION('19.1.0')
OPTIMIZER_FEATURES_ENABLE('19.1.0')
IGNORE_OPTIM_EMBEDDED_HINTS
END_OUTLINE_DATA
*/
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter( EXISTS (SELECT 0 FROM "T2" "T21" WHERE "T21"."ID"=:B1))
5 - filter("T21"."ID"=:B1)
6 - filter( NOT EXISTS (SELECT 0 FROM "T2" "T22" WHERE
"T22"."ID"=:B1))
8 - filter("T22"."ID"=:B1)
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
68 rows selected.
The gather_plan_statistics hint was not really helpful as it is a nested cursor, so that it does not show the rowsource statistics from that level - the nested cursor runs separately and its rowsource statistics are not included in the top-level cursor ones:
-----------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 |
| 1 | VIEW | | 0 | 2 | 0 |00:00:00.01 |
| 2 | UNION-ALL | | 0 | | 0 |00:00:00.01 |
|* 3 | FILTER | | 0 | | 0 |00:00:00.01 |
| 4 | TABLE ACCESS FULL| T1 | 0 | 1 | 0 |00:00:00.01 |
|* 5 | TABLE ACCESS FULL| T2 | 0 | 1 | 0 |00:00:00.01 |
|* 6 | FILTER | | 0 | | 0 |00:00:00.01 |
| 7 | TABLE ACCESS FULL| T1 | 0 | 1 | 0 |00:00:00.01 |
|* 8 | TABLE ACCESS FULL| T2 | 0 | 1 | 0 |00:00:00.01 |
| 9 | FAST DUAL | | 1 | 1 | 1 |00:00:00.01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter( IS NOT NULL)
5 - filter("T21"."ID"=:B1)
6 - filter( IS NULL)
8 - filter("T22"."ID"=:B1)
I found the explanation of this issue in the SQL trace file:
=====================
PARSING IN CURSOR #139973723001152 len=579 dep=0 uid=78 oct=3 lid=78 tim=4622189819 hv=2185287422 ad='68d3c950' sql
id='0mkkpha141pry'
select cursor(
select query_type
from (select 'A' query_type
from t1 t11
where exists (
select null
from t2 t21
where t21.id = t11.id)
union all
select 'B' query_type
from t1 t12
where not exists (
select null
from t2 t22
where t22.id = t12.id)
)
) data
from dual
END OF STMT
PARSE #139973723001152:c=11048,e=12511,p=0,cr=45,cu=12,mis=1,r=0,dep=0,og=1,plh=2197603499,tim=4622189819
EXEC #139973723001152:c=13,e=13,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=2197603499,tim=4622189873
=====================
PARSING IN CURSOR #139973720520408 len=272 dep=2 uid=78 oct=3 lid=78 tim=4622190302 hv=3362864010 ad='68d141b0' sqlid='faj0udr472fwa'
SELECT "A2"."QUERY_TYPE" "QUERY_TYPE"
FROM (
(SELECT 'A' "QUERY_TYPE"
FROM "T1" "A5"
WHERE EXISTS (
SELECT 0
FROM "T2" "A6"
WHERE "A6"."ID"=:CV1$))
UNION ALL
(SELECT 'B' "QUERY_TYPE"
FROM "T1" "A4"
WHERE NOT EXISTS (
SELECT 0
FROM "T2" "A7"
WHERE "A7"."ID"=:CV2$))) "A2"
END OF STMT
PARSE #139973720520408:c=375,e=375,p=0,cr=0,cu=0,mis=1,r=0,dep=2,og=1,plh=0,tim=4622190302
..skip..
BINDS #139973720520408:
Bind#0
oacdty=02 mxl=22(00) mxlc=00 mal=00 scl=00 pre=00
oacflg=11 fl2=0001 frm=00 csi=00 siz=48 off=0
kxsbbbfp=7f4e2be3b278 bln=22 avl=00 flg=05
Bind#1
oacdty=02 mxl=22(00) mxlc=00 mal=00 scl=00 pre=00
oacflg=11 fl2=0001 frm=00 csi=00 siz=0 off=24
kxsbbbfp=7f4e2be3b290 bln=22 avl=00 flg=01
EXEC #139973720520408:c=9732,e=9360,p=0,cr=37,cu=0,mis=1,r=0,dep=2,og=1,plh=489056501,tim=4622199711
FETCH #139973723001152:c=10218,e=9845,p=0,cr=37,cu=0,mis=0,r=1,dep=0,og=1,plh=2197603499,tim=4622199753
STAT #139973723001152 id=1 cnt=0 pid=0 pos=1 obj=0 op='VIEW (cr=0 pr=0 pw=0 str=0 time=0 us cost=12 size=6 card=2)'
STAT #139973723001152 id=2 cnt=0 pid=1 pos=1 obj=0 op='UNION-ALL (cr=0 pr=0 pw=0 str=0 time=0 us)'
STAT #139973723001152 id=3 cnt=0 pid=2 pos=1 obj=0 op='FILTER (cr=0 pr=0 pw=0 str=0 time=0 us)'
STAT #139973723001152 id=4 cnt=0 pid=3 pos=1 obj=25013 op='TABLE ACCESS FULL T1 (cr=0 pr=0 pw=0 str=0 time=0 us cost=3 size=13 card=1)'
STAT #139973723001152 id=5 cnt=0 pid=3 pos=2 obj=25014 op='TABLE ACCESS FULL T2 (cr=0 pr=0 pw=0 str=0 time=0 us cost=3 size=13 card=1)'
STAT #139973723001152 id=6 cnt=0 pid=2 pos=2 obj=0 op='FILTER (cr=0 pr=0 pw=0 str=0 time=0 us)'
STAT #139973723001152 id=7 cnt=0 pid=6 pos=1 obj=25013 op='TABLE ACCESS FULL T1 (cr=0 pr=0 pw=0 str=0 time=0 us cost=3 size=13 card=1)'
STAT #139973723001152 id=8 cnt=0 pid=6 pos=2 obj=25014 op='TABLE ACCESS FULL T2 (cr=0 pr=0 pw=0 str=0 time=0 us cost=3 size=13 card=1)'
STAT #139973723001152 id=9 cnt=1 pid=0 pos=2 obj=0 op='FAST DUAL (cr=0 pr=0 pw=0 str=1 time=0 us cost=2 size=0 card=1)'
FETCH #139973720520408:c=178,e=177,p=0,cr=8,cu=0,mis=0,r=1,dep=0,og=1,plh=489056501,tim=4622200599
STAT #139973720520408 id=1 cnt=1 pid=0 pos=1 obj=0 op='VIEW (cr=9 pr=0 pw=0 str=1 time=179 us cost=12 size=6 card=2)'
STAT #139973720520408 id=2 cnt=1 pid=1 pos=1 obj=0 op='UNION-ALL (cr=9 pr=0 pw=0 str=1 time=177 us)'
STAT #139973720520408 id=3 cnt=0 pid=2 pos=1 obj=0 op='FILTER (cr=1 pr=0 pw=0 str=1 time=8 us)'
STAT #139973720520408 id=4 cnt=0 pid=3 pos=1 obj=25013 op='TABLE ACCESS FULL T1 (cr=0 pr=0 pw=0 str=0 time=0 us cost=3 size=0 card=1)'
STAT #139973720520408 id=5 cnt=0 pid=3 pos=2 obj=25014 op='TABLE ACCESS FULL T2 (cr=1 pr=0 pw=0 str=1 time=6 us cost=3 size=13 card=1)'
STAT #139973720520408 id=6 cnt=1 pid=2 pos=2 obj=0 op='FILTER (cr=8 pr=0 pw=0 str=1 time=164 us)'
STAT #139973720520408 id=7 cnt=1 pid=6 pos=1 obj=25013 op='TABLE ACCESS FULL T1 (cr=7 pr=0 pw=0 str=1 time=144 us cost=3 size=0 card=1)'
STAT #139973720520408 id=8 cnt=0 pid=6 pos=2 obj=25014 op='TABLE ACCESS FULL T2 (cr=1 pr=0 pw=0 str=1 time=9 us cost=3 size=13 card=1)'
FETCH #139973723001152:c=0,e=0,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=0,plh=2197603499,tim=4622200901
CLOSE #139973720520408:c=3,e=2,dep=1,type=0,tim=4622201312
CLOSE #139973723001152:c=6,e=193,dep=0,type=0,tim=4622201500
That is where the most remarkable part happens. The subquery is modified in an awfully wrong way: the columns of the outer queries disappear from the inner queries, and they are replaced with some bind variables: CV1$, CV2$.Those variables, which are of the NUMBER data type, are set to NULL.
Then, the 'A' branch of the UNION ALL, which uses EXISTS, returns nothing. The 'B' branch, which uses NOT EXISTS, returns the 'B' row since the NOT EXISTS subquery brings back an empty result set.
At this stage the root cause of the wrong results issue is known. Obviously, I have no clue why those variables were introduced but I know why that query returns that result.
I tried to tinker around with that query a little bit to see what can be done to make it work, and here is a short summary of my findings:
- Correlated subqueries are essential. The query starts returning the correct results (one 'A' row) once I decorrelate the subqueries:
SQL> select cursor( 2 select query_type 3 from (select 'A' query_type 4 from t1 t11 5 where id in ( 6 select id 7 from t2 t21) 8 union all 9 select 'B' query_type 10 from t1 t12 11 where id not in ( 12 select id 13 from t2 t22) 14 ) 15 ) data 16 from dual; DATA -------------------- CURSOR STATEMENT : 1 CURSOR STATEMENT : 1 Q - A - Nesting of the subqueries does matter. For instance, the query returns the correct results after I reduce the nesting level:
SQL> select cursor( 2 select 'A' query_type 3 from t1 t11 4 where exists ( 5 select null 6 from t2 t21 7 where t21.id = t11.id) 8 union all 9 select 'B' query_type 10 from t1 t12 11 where not exists ( 12 select null 13 from t2 t22 14 where t22.id = t12.id) 15 ) data 16 from dual; DATA -------------------- CURSOR STATEMENT : 1 CURSOR STATEMENT : 1 Q - A
- Less rows:
SQL> doc DOC>############################## DOC> Less rows DOC>############################## DOC># SQL> SQL> select cursor( 2 select query_type 3 from (select 'A' query_type 4 from t1 t11 5 where exists ( 6 select null 7 from t2 t21 8 where t21.id = t11.id) 9 union all 10 select 'B' query_type 11 from t1 t12 12 where exists ( 13 select null 14 from t2 t22 15 where t22.id = t12.id) 16 ) 17 ) data 18 from dual; DATA -------------------- CURSOR STATEMENT : 1 CURSOR STATEMENT : 1 no rows selected SQL> SQL> doc DOC> The cursor subquery DOC># SQL> select query_type 2 from (select 'A' query_type 3 from t1 t11 4 where exists ( 5 select null 6 from t2 t21 7 where t21.id = t11.id) 8 union all 9 select 'B' query_type 10 from t1 t12 11 where exists ( 12 select null 13 from t2 t22 14 where t22.id = t12.id) 15 ) 16 ; Q - A B - More rows:
SQL> doc DOC>############################## DOC> More rows DOC>############################## DOC># SQL> SQL> select cursor( 2 select query_type 3 from (select 'A' query_type 4 from t1 t11 5 union all 6 select 'B' query_type 7 from t1 t12 8 where not exists ( 9 select null 10 from t2 t22 11 where t22.id = t12.id) 12 ) 13 ) data 14 from dual; DATA -------------------- CURSOR STATEMENT : 1 CURSOR STATEMENT : 1 Q - A B SQL> SQL> doc DOC> The cursor subquery DOC># SQL> select query_type 2 from (select 'A' query_type 3 from t1 t11 4 union all 5 select 'B' query_type 6 from t1 t12 7 where not exists ( 8 select null 9 from t2 t22 10 where t22.id = t12.id) 11 ); Q - A - Different values as it was demostrated in the beginning of this post
The bad news is that the issue is not mentioned on the document I would expect it to be: Things to Consider to Avoid Prominent Wrong Result Problems on 19C Proactively (Doc ID 2606585.1). How is one supposed to avoid such issues? I always read those 'things to consider' before I devise any upgrade plans.
The scripts, a sample trace file, and the spool file for this post are available on: GitHub.
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